1. Field of the Invention
The invention relates generally to analog-to-digital and digital-to-analog conversion systems associated with proper scaling before A/D conversion and after D/A conversion, and particularly to such a system for an echo canceller.
2. Prior Art
Analog-to-digital (A/D) and digital-to-analog (D/A) converters have come widely into use because of the development of digital processing systems. For instance, in the technical field of control and monitoring systems, that is devices for maintaining conditions in operating systems as close as possible to desired values despite changes in the operating environment, analog components have traditionally been used. In the 1970's, the use of discrete or logical control elements and programmable logic controllers became widespread. This, in turn, allowed the development of digital control systems for monitoring such things as chemical processes, machine tools and other industrial processes and operations. To achieve this goal, analog-to-digital (A/D) converters are used to transform analog information, such as audio signals or measurements of physical variables (for example, temperature, force, or electric voltage) into a form suitable for digital handling. FIG. 1A illustrates a conventional case of process control involving analog components only in which the use of negative feedback provided by block 3 produces changes in the characteristics of the system which improve the performance of the system. In this automatic control system, feedback is used to compare, by means of subtracter 1, the actual output S of A system with a desired input E, the difference appearing at the output of subtracter 1 being used as the input signal of a controller 2. A high performance of the system in terms of dynamic performance and stability often involves a sophisticated feedback function in block 3 which may advantageously be designed by means of precise and cheap digital processing systems, as in FIG. 1B. However, when both output S and input E of the system are analog variables, the use of digital technology for providing the feedback function in block 4 involves the conversion of the first variable into digital form prior to any computation, and then the conversion of the digital output of block 4 into analog form in order to generate the analog variable E' that will be eventually used to produce the difference E-E'. However, in some environment of changing characteristics, the level of the electric voltage S to be converted is likely to vary significantly in a wide range. It is therefore essential to perform a proper scaling of the analog signal S prior to its conversion, in order to make the best use of the A/D converter precision. This A/D conversion is therefore performed after an amplification step in block 6. After computation in block 4, the digital result is converted back into its analog form in block 5 which performs the D/A conversion and then an attenuation for providing the analog value E'. In order to allow a high performance of the system, the precise mastering of the transfer functions of block 4, 5 and 6 is most desirable and particularly, both transfer function of blocks 5 and 6, i.e. both A/D-amplification and D/A-attenuation processings, should be accurately inverse of each other.
Similarly, echo cancellation techniques allowing high speed full-duplex data communication on a single channel also require precise transfer functions included into the echo cancelling loop in order to achieve high rejection ratios for the echo. The cancellation of the echo is achieved by an echo estimator which generates an estimation of the value of the echo signal that spoils the received signal from a far-end data control equipment such as a modem. The estimated echo is subtracted from the received signal in order to produce a signal being as close as possible to the ideal received signal having no echo. The estimation of the echo by means of digital processing systems in a standard analog 4-wire modem again involves two accurate A/D-amplification and D/A-attenuation transfer functions. This particular case will be described in detail with respect to FIGS. 7A, 7B and the figures following them.
In the background art, the design of precise transfer functions, and particularly two transfer functions being accurately inverse of one another for performing a A/D conversion associated with an amplification step, and a D/A conversion associated with a attenuation step have always involved adjustable components and precise elements which inevitably increase the final cost of the system. Moreover, even precise adjustments can not guarantee accurate transfer functions since the values of the components are subject to long-term shift.